1. Description of the Prior Art
In recent years various low-alloy high-strength steel sheets have been under development, and the present inventors have also invented a high-silicon and high-manganese steel ("AN ISOTROPIC AND HIGH-STRENGTH HIGH-SILICON STEEL SHEET" U.S.P. Ser. No. 477,098, British Patent Application No. 25798/74, German Patent Application P 24 27 837.3, French Patent Application No. 74/19982, Italian Patent Application No. 23832A/74, Swedish Patent Application No. 74 07542-5 and Brazilian Patent Application No. 4792P/74).
The high-silicon and high-manganese steel of U.S.P. Ser. No. 477,098 comprises 0.03 to 0.15% carbon, 0.7 to 2.3% silicon and 0.7 to 2.0% manganese, the balance being iron and unavoidable impurities. The ratio of silicon/manganese in this steel is between 0.6 and 1.5, preferably between 0.8 and 1.1, and the content of sulfur is maintained as low as possible below that usually present as an unavoidable impurity, preferably not more than 0.01%. This steel may also contain one or more of Cr, Nb, V, Ti, Al and Cu in amounts of not more than 0.5% for Cr and Cu, not more than 0.4% for each of Nb, V and Ti, and not more than 0.1% for Al. Further, this steel may contain Ce and/or Zr in an amount so as to attain a Ce/S ratio of 1.5 to 2.0 and a Zr/S ratio of greater than 2.
This steel is a high-silicon and high-manganese steel useable as both hot and cold rolled steel sheets, in which the weight percent ratio of Si/Mn is about 1 : 1 and which shows a ductility and concomitant tensile strength superior to that of various high-strength steel sheets conventionally available. Especially noticeable is that the steel in the state of "dual-phase" structure, consisting of ferrite- and martensite (and/or bainite) phases, formed after continuous annealing in the alphagamma temperature range followed by air cooling, provides a high-strength high-ductility steel sheet of up to 90 kg/mm grade in tensile strength. The dual-phase steel is characteristic of low yield strength (which can easily be raised by a low-temperature reheating), high tensile strength, a good elongation.
As for a somewhat similarly structured steel, Japanese Patent Publication No. Sho 46-9542 (U.S. Pat. No. 3,378,360) (A METHOD FOR PRODUCING A HIGH TENSILE STRENGTH THIN STEEL SHEET) may be mentioned.
According to the method disclosed by this prior art, it is essential that very rapid cooling such as water quenching from the alpha-gamma range is carried out [within 0.8 second from the alpha-gamma range to the Ms (start of martensitic transformation) temperature, hence with a cooling of about 3000.degree.-40000.degree. C/min.] in a continuous annealing process in order to assure a high level of strength.
Such drastic cooling as by water quenching requires additional equipment and increased production cost, and causes further trouble such that the shape of the steel sheet tends to be deformed due to the severe thermal contraction strain, requiring proper shape-correction devices. In addition to these, it is rather difficult to attain a high level of ductility by the water-quenching method, even though a high level of strength can be easily obtained. Thus it has been generally difficult to obtain a good combination of high strength (up to about 80 kg/mm.sup.2) practically applicable to press forming and high ductility.
Examples in Table 1 illustrate these high-strength low-ductility materials.
Table 1 __________________________________________________________________________ Strength and Ductility of High-Strength Steel Sheets obtained by the Alpha-Gamma-Range Heating and Water Quenching by Prior Art (According to Japanese Patent Publication No. Sho 46-9542; U.S. Pat. No. 3,378,360) Yield Sheet Strength Thick- (0.2 Proof Tensile Total Composition (weight %) ness Stress) Strength Elongation C Si Mn P S mm kg/mm.sup.2 kg/mm.sup.2 % __________________________________________________________________________ 0.08 0.006 0.43 0.009 0.034 0.165 63.3 72.1 3.0 0.12 0.009 0.47 0.009 0.025 0.173 68.1 75.9 3.0 0.08 0.004 0.41 0.016 0.030 0.165 55.7 71.2 6.5 __________________________________________________________________________ Remarks: .sup.1 Starting Material; A carbon steel strip coil containing cementite particles formed by repetition of cold working and annealing, and containing no pearlite. .sup.2 Treatment; Heated in the alpha-gamma range, i.e. between A.sub.1 and A.sub.3 transformation points, and water quenched (estimated cooling velocity would be 30000 - 40000.degree. C/min.).
As another prior art for production of this kind of steel, Japanese Patent Publication Sho 31-1303 (A PROCESS FOR PRODUCING "MARTENO-FERRITE" STEEL) may be mentioned, according to which it is essential to heat a ferrite-pearlite steel with a heating rate of at least 100.degree. C/sec. (6000.degree. C/min.) up to the completion of the Ac.sub.1 transformation, and it is also essential to introduce drastic quenching immediately. However, such a rapid heating can be realized only in a laboratory using a very small test piece and an appropriate heating means, but it is almost impossible to apply such a rapid heating to the commercial production of press-formable steel sheets by a continuous annealing apparatus.
The extremely rapid heating and cooling required by the above prior arts are essential for preventing the steel from taking the heat cycle to and from the alpha-gamma range in a quasi-equilibrium manner (namely for preventing carbon diffusion accompanying the heat cycle) so as to attain martensitic transformation of the carbon concentrated phase formed in the alpha-gamma range. Therefore, if it is possible to process a steel through the heat cycle (with mild rates of heating and cooling) in a non-equilibrium manner (sufficient to cause the eventual matensitic transformation) in respect to carbon diffusion by a practical and feasible metallurgical means, as a substitute for the severe rapid heating and cooling, it is possible to realize a ferrite-martensite dual-phase steel with a practically applicable heating rate and a cooling rate mild enough to eliminate both the shape deterioration and large capical investment. Naturally it is necessary that the transformed phase after cooling is uniformly dispersed in view of mechanical isotropy and good elongation. If a dual-phase steel satisfying the above conditions can be realized, remarkable industrial advantages can be assured for the production of a low-cost high-strength and high-ductility steel sheet.
The present inventors have solved the above problem based on the discovery of an optimum value of the proportions of Si and Mn in the high-silicon high-manganese dual-phase steel previously invented by the present inventors. Namely the present inventors completed the prior invention: (1) on the basis of the fact that an isotropic dispersion of the pearlite prior to the alpha-gamma-range heating can be obtained by appropriate proportions of Si and Mn, and (2) on the basis of the consideration that Si would be effective for controlling the carbon diffusion, preventing the austenite decomposition into pearlite. As a result, the present inventors succeeded in the prior invention to obtain a ferrite-martensite dual-phase structure after cooling from the alpha-gamma range, without drastic quenching (any heating rate is applicable and any cooling rate not lower than 100.degree. C/min. is applicable), with the remarkable advantages that the ductility is excellent within a tensile strength range up to about 90 kg/mm.sup.2, and that the remarkable increase in yield strength can be obtained by a low temperature reheating treatment or by baking for surface coating treatment after pressing. The advantage that such a structure and properties can be obtained by a relatively slow cooling following the alpha-gamma-range heating is based on an appropriate content of carbon as well as appropriate content of silicon and manganese.
The prior invention made by the present inventors is an excellent method for obtaining a good combination of high strength and high ductility, applicable to rather thick cold rolled sheets, up to about 2 mm, as well as to a thin steel sheet of less than about 1 mm thickness.
However, the present inventors have found through experiments that the relatively high content of silicon in the prior invention causes some problems in the production process, such as deterioration of the nature of scale at the rough hot rolling, even though it is not too difficult to commercially produce the objective products. It is also desirable to lower the silicon content from a point of view of production cost. Further, this prior invention has some problems due to high silicon content, regarding adhesivity of inorganic phosphate coatings applied onto the cold rolled sheets.
When the silicon content is lowered, it is very natural that controlling effect on the carbon diffusion is lowered. However, if the steel sheet is thin enough the air cooling rate becomes considerably large so that it is expected that pearlite formation (due to carbon diffusion) by the decomposition of the austenite phase formed in the alpha-gamma range is likely to be prevented. Therefore, it is considered that isotropic dispersion of the carbon concentrated phases (pearlite and/or carbide phases) prior to the alpha-gamma-range heating must be attained by some means other than the high silicon content.
Demands have been made for development of a method for producing a steel sheet (of dual-phase structure) having a good combination of strength and ductility properties, similar to those obtained by the prior invention made by the present invention, from a steel composition of low silicon content as in ordinary Si-Mn steels, particularly for the object of producing a high strength steel sheet of small thickness (about 1 mm) for forming.
The present invention has been completed to meet the above demand.